1. Field of the Invention
The present invention relates to a photoelectric converter that receives a light reflected from an original to which the light is irradiated, and converts the light into an electric signal, and more specifically to a linear image sensor suitable for an image reading device such as a facsimile or an image scanner.
2. Description of the Related Art
A circuit diagram of an image sensor IC used in a conventional image reading device is shown in FIG. 4, and a timing chart thereof is shown in FIG. 5. The image sensor is disclosed in Japanese Patent Application Laid-open No. Hei 10-051164.
An n-type region of a photo diode 101 is connected to a positive supply voltage terminal VDD, and a p-type region is connected to a drain of a reset switch 102 and a gate of a source follower amplifier 103. A source of the reset switch 102 is given a reference voltage VREF1. A source that is an output terminal of the source follower amplifier 103 is connected to a read switch 105 and a constant current source 104. The gate of the constant current source 104 is given a constant voltage of a reference voltage VREFA. The number of elements disposed inside of a frame of a photoelectric conversion block An shown in FIG. 4 is identical with the number of pixels, and the read switches 105 of the respective blocks are connected to a common signal line 106. Note that the photoelectric conversion block An is indicative of a photoelectric conversion block of an n-th bit.
The common signal line 106 is inputted to an inverse terminal of an operational amplifier 109 through a resistor 110, and an output terminal of the operational amplifier 109 is connected to an output terminal 116 through a chip select switch 112 and a capacitor 113. The common signal line 106 is connected to a signal line reset switch 107, and a source of the signal line reset switch 107 is given a reference voltage VREF2. A resistor 111 is connected between the output terminal and the inverse terminal of the operational amplifier 109, and a non-inverse terminal of the operational amplifier 109 is fixed to a constant voltage VREF3. An inverse amplifier D is composed of the operational amplifier 109, the resistor 110 and the resistor 111.
An output terminal 116 of the image sensor is connected to a drain of the MOS transistor 114, and a source of the MOS transistor 114 is given a reference voltage VREF4. Also, the output terminal 116 of the image sensor is also connected to a capacitor 115 such as a parasitic capacitor. A clamp circuit C is composed of the capacitor 113, the capacitor 115 and the MOS transistor 114.
However, in the image sensor of the above type, the photo diode is reset after an optical signal voltage is read subsequent to the completion of photocharge storage, and thereafter the reference voltage is read, and a difference between the optical signal voltage and the reference voltage is taken. This leads to such a problem that reset noises put on the reference voltage and the optical signal voltage are different from each other. That is, because the reset noises of the different timings are compared with each other, there arises a problem in that the random noises are large.
In order to solve the above-mentioned problems inherent in the prior art, according to the present invention, there is provided a photoelectric converter in which: an output terminal of a photoelectric converter means is connected to input terminals of a reset means and an amplifying means; a charge transfer means is connected to an output terminal of the amplifying means; another terminal of the charge transfer means is connected to a capacitor and a gate of a source follower amplifier; a source of the source follower amplifier is connected to a channel select means; another terminal of the channel select means is connected to a common signal line; and the common signal line is connected to a current source, and the photoelectric converter is characterized in that: the channel select means is closed to read a reference voltage in the common signal line; the charge transfer means is closed to read signal charges in the capacitor; the charge transfer means is opened after the channel select means is opened; and the channel select means is closed to read a signal voltage in the common signal line; the reset means is closed to initialize the photoelectric conversion means; the charge transfer means is closed to read reference charges in the capacitor after the reset means is opened; and the charge transfer means is opens after the channel select means is opened.
Alternatively, the photoelectric converter according to the present invention is characterized in that: the channel select means is closed to read a reference voltage in the common signal line; the charge transfer means is closed to read signal charges in the capacitor; the charge transfer means is closed to read signal charges in the capacitor; the charge transfer means is opened to read a signal voltage in the common signal line; the reset means is closed to initialize the photoelectric conversion means; the charge transfer means is closed to read reference charges in the capacitor after the reset means is opened; and the channel select means is opened after the charge transfer means is opened.
Also, there is provided a photoelectric converter, in which: an output terminal of a photoelectric converter means is connected to input terminals of a reset means and a first amplifying means; a first charge transfer means is connected to an output terminal of the first amplifying means; another terminal of the first charge transfer means is connected to input terminals of a first capacitor and a second amplifying means; a second charge transfer means is connected to an output terminal of the second amplifying means; another terminal of the second charge transfer means is connected to input terminals of a second capacitor and a third amplifying means; a third charge transfer means is connected to an output terminal of the third amplifying means; another terminal of the third charge transfer means is connected to a third capacitor and a gate of a source follower amplifier; a source of the source follower amplifier is connected to a channel select means; another terminal of the channel select means is connected to a common signal line; and the common signal line is connected to a current source, characterized in that a following series of operations is repeated, in which: the second charge transfer means operates; subsequently, the first charge transfer means and the third charge transfer means operate; subsequently, the reset means and the second charge transfer means operate; subsequently, the first charge transfer means operates; subsequently, the channel select means operates; subsequently, the third charge transfer means operates; and subsequently, the channel select means operates.
According to this reading manner, since the reference voltage and the signal voltage containing the same off-noise of the reset switch are read in order, if a voltage difference is taken by using the correlated double sampling circuit or the like, a photoelectric converter small in the fixed pattern noise and the random noise can be obtained.